Eu \Eu\ [Gr. ? well, orig. neut. of ? good; prob. connected with Skr. su, from the same root as E. is; or with Skr. vasu good, prob. fr. the same root as E. was.]

A prefix used frequently in composition, signifying well, good, advantageous; -- the opposite of dys-.

Source: Webster's Revised Unabridged Dictionary (1913)